The present invention is directed to a toric pump having an improved impeller which minimizes internal leakage through the clearance gap between the impeller and pump housing and which minimizes the noise generated by operation of the pump.
Toric pumps of the type with which the present invention is concerned employ a disk-like impeller having a series of radial vanes mounted around its periphery. The opposed side surfaces of the impeller are flat, except for pockets between the vanes, and the impeller is mounted within a pump housing having an internal chamber having opposite side surfaces and a peripheral surface which closely enclose the impeller but allows sufficient clearance such that the fluid can exit the impeller radially and then turn forward or backward into the internal pump chambers of the housing. The chamber walls are formed with an internal pump chamber or passage extending along an annular path in operative relationship with the path of the impeller vanes at as constant radial distance from the impeller axis from an inlet at one end of the toroidal passage to an outlet at the opposite end. The circumferential extent of the toroidal passage around the pump axis is less than 360.degree., and between the ends of the passage a relatively narrow portion of the chamber side wall extends across the annular region traversed by the toroidal chamber. This portion of the chamber side wall is called the stripper and the stripper functions to deflect fluid being impelled through the pump chamber by the impeller vanes into the pump outlet instead of being pumped back to the inlet.
During operation of the pump, as each vane advances past the outlet end of the pump chamber to cross the stripper, the sudden reduction in the cross sectional area of the chamber through which the vane is moving generates a discontinuity in the fluid flow. Such a discontinuity occurs each time a vane passes across an edge of the stripper and, there is thus a generation of a cyclic change of resistance to the rotation of the impeller. Where the vanes are equally spaced around the impeller periphery, the frequency of this cyclic reaction is directly proportional to the rotative speed of the impeller, and at certain critical speeds, structural resonances or harmonics may develop which generate noise. It has been recognized in the prior art that this problem may be solved to some extent by varying the vane spacing around the periphery of the impeller. However, variable vane spacing usually results in the creation of at least some rotor imbalance which in turn leads to problems potentially more serious than undesirable noise.
A second problem encountered by pumps of types described above results from the fact that a slight clearance or gap must exist between the stationary pump housing surfaces and the adjacent rotating surfaces of the impeller in order that the impeller can freely rotate relative to the housing. Those portions of the chamber side surfaces and the opposed side surfaces of the impeller which are located radially inwardly of the toroidal pump chamber present a gap which extends the entire length of the radially inner side of the circumferentially extending pump chamber. Pressure progressively increases in this chamber from the inlet end to the outlet end, and the clearance gap provides a path for leakage of fluid from high pressure regions of the chamber to regions of lower pressure. Where the fluid being pumped is of low viscosity--i.e., air for example--this leakage can be substantial and substantially reduce the flow delivered by the pump.
Prior art attempts to employ a labyrinth type seal to reduce this leakage have not, in general been successful as demonstrated by the fact that very few, if any, commercially available regenerative pumps employ such seals. Labyrinth seals rely upon a series of restrictions separated by expansion chambers which are intended to enable the fluid entering the chamber to expand to an increased volume or bulk which is in theory more difficult to pass through the next following restriction. Where the fluid is of low compressibility, such as a liquid, no expansion takes place and the presence of the expansion chambers reduces the area available for restriction, thus reducing the effectiveness of the seal. Where the pump of the type described above is employed to pump gases, the gasses are highly compressible, but the pumps typically develop only a relatively small pressure differential between the inlet and outlet. Because of the relatively small differential between the density of the compressible fluid at the inlet and its density at the outlet, there is little opportunity for expansion of the gas in the expansion chambers of a labyrinth seal. Further, most of the prior art effort has focused on reducing leakage across the stripper between the inlet and outlet ends of the chamber while ignoring the fact that leakage likewise may occur between points in the chamber which are not necessarily closely adjacent the inlet or outlet.
The present invention is directed to a solution of the problems discussed above.